1. Technical Field
The present invention relates to coded modulation and more particularly to polarization-division multiplexed (PDM) discrete-time non-binary low-density parity-check-coded modulation (NB-LDPC-CM).
2. Description of the Related Art
Due to the rapid growth of data-centric services and the general deployment of broadband access networks in recent years, there has been an elevated demand driving the dense wavelength division multiplexing (DWDM) network upgrade from 10 Gb/s per channel to more spectrally-efficient channel transmission rates. However, as the symbol rate increases, the deteriorating effects of linear and nonlinear fiber impairments are known to exacerbate. For example, as the communication rate over a given medium increases, transmission becomes increasingly sensitive to errors due to various linear and nonlinear channel impairments such as chromatic dispersion, PMD and fiber nonlinearities. The Shannon limit for a noise-influenced channel describes a maximum amount of error-free data that can be transmitted with a specified bandwidth—it is therefore helpful to have robust codes and modulation schemes that closely approach the Shannon limit without imposing high requirements in terms of implementation cost and complexity.
Bit interleaved (BI) low-density parity-check (LDPC) coded modulation (CM) based on large girth LDPC codes provides excellent performance, but requires code rate and bandwidth to increase in order to compensate for information loss due to coding. Meanwhile, quasi-cyclic (QC) LDPC codes are easy to implement, but this comes at the expense of performance. Large-girth QC-LDPC codes provide good bit-error rate (BER) performance, but require excessive codeword length for larger girths, as the code rate, and therefore the bandwidth, must increase to compensate for information loss due to coding. Another approach used to enable higher speed data transport is the concatenation of trellis-coded modulation (TCM) (initially introduced for wire-line transmissions) with an outer interleaved Bose-Chaudhuri-Hocquenghem (BCH) code. However, as discussed further below, the performance of systems using BCH-TCM lags far behind that of systems using LDPC codes, at least in part because those systems use weak convolutional codes.